In multicolor web-fed printing press systems, a web of material (e.g., paper) is sequentially driven through a series of printing units, each comprising a plate cylinder and a print cylinder (blanket cylinder). Each blanket cylinder contacts the web in sequence and applies a different color of ink thereto, which colors cooperate to imprint a multicolor image on the web. As the web exits the printing units, the ink is still wet, and thus subject to smearing. Accordingly, for further processing, the web is typically routed through a drying unit to dry the image, heating the web to evaporate various solvents in the ink, then to a chill roller unit to cool the web and harden the ink.
To provide an accurate and clear multicolor image, the rotational and lateral position of each blanket cylinder must be precisely aligned, i.e., proper registration of the respective colors must be maintained. Sources of inaccurate color registration include web "weave" (spurious lateral movement of the web, e.g., movement transverse to the direction of web travel, in the plane of the web) and web "flutter" (spurious movement of the web in a direction perpendicular to the plane of the web).
Other factors may also affect print quality. More particularly, the printed web may display streaking marks as the web exits the chill roller unit. It is generally accepted that this web streaking problem, commonly called "chill streaking" or "condensate marking", is caused by the formation of a contaminant condensate film typically on the first roller of the chill roller unit.
In order to dry the image printed on a web, the drying unit heats the web which evaporates various solvents in the ink. The majority of the contaminated warm air, which contains evaporated ink solvents and gases from the combustion in the drying unit as well as evaporated moisture from the web, is directed through an exhaust system comprising pollution control devices designed to eliminate the contaminants from the warm air before exhausting it to the outside. However, part of the contaminated air is not processed through this exhaust system. This is because as the web exits the drying unit, a boundary layer of contaminated warm air, which adheres to the upper and lower surfaces of the web, is entrained by the web toward the next processing station, namely, the chill unit. As the web engages the first roller of the chill roll unit, the contaminated warm air may become trapped between the surface of the web and that of the cool chill roller or may condense on the relatively cool surface of the chill roller. As a result, a condensate film, containing contaminants, forms on the surface of the chill roller. This condensate, which is in direct contact with the web, is the source of the "chill streaking" problem commonly occurring in such printing press systems where the image on a web is dried by a hot drying process.
To eliminate the formation of streaking marks on the web, a commonly used approach has been to reduce the speed at which the press operates thereby reducing the amount of contaminants entrained by the web out of the drying unit. Although this approach is successful in most cases, it is, for economical reasons, highly undesirable since throughput of the printing press system is thereby reduced.
Another method to reduce chill streaking consists of increasing the tension to which the web is subjected by the printing press so as to assure a more uniform contact between the web and the chill roller. Under increased web tension, it becomes more difficult for the contaminated air to "lift" the web off the chill roller and, accordingly, condensation on the chill roller is reduced. This method, which gives adequate results under uniform web characteristics, is of limited effectiveness in practice since web characteristics generally vary over a given press run. Accordingly, as the web stretches, a gap will appear between the surface of the web and the surface of the chill roller, allowing condensation to form therebetween. Conversely, increasing web tension to eliminate the gap between the chill roller and the web in order to reduce chill marking increases the likelihood of web breakage.
Other approaches have been tried to eliminate chill marking by either invasively removing the condensate deposit from the chill roller, as by wiping, or by preventing formation of condensate while keeping the press operating under normal speed and web tension conditions. An example of a system using the former approach is illustrated in a sales brochure entitled "Chill Roll Cleaner, Model 1301," by Baldwin.
FIG. 2 shows a section view of a prior art chill roll cleaner representative of the Model 1301 Baldwin device. In FIG. 2, a chill roll cleaner 217 is mounted on a first chill roller 115 of a chill unit 114 and continuously cleans first chill roller 115 by pressing an absorbent material 223 against the surface of chill roller 115. Absorbent material 223 of chill roll cleaner 217 is dispensed by a feed roller 219. Soiled absorbent material 223 is collected over a collect roller 221. The frequency of advance of collect roller 221 is adjusted by the pressman as necessary to achieve adequate cleaning of chill roller 115.
Such an invasive system offers increased safety and some degree of automation over manual cleaning, since manual cleaning requires the pressman, during operation of the chill unit, to manually sweep the condensate film off the chill roller. However, invasive prior art systems have disadvantages. First, as with manual sweeping, the condensate film is removed through an invasive process, that is, a cleaning material makes direct contact with the surface of the chill roller. Direct contact with the surface of a chill roller increases the risk of damaging the chill roller as dust particles trapped between the cleaning material and the chill roller are continually dragged over the same area of the chill roller surface, eventually leading to a press shut-down to resurface or replace a damaged chill roller. Second, such an invasive chill roll cleaner system generally results in additional or longer press down-time when a new cleaning material feed roller needs to be installed. Finally, special procedures for proper disposal of soiled cleaning material must be followed as the condensate, which contains ink solvents and combustion products may be considered a toxic waste.
Another attempt to deal with the chill streaking problem has been to prevent formation of the condensate deposit on the chill roller without increasing web tension as by forcing web-to-roll contact. An example of a system using this forced contact approach is illustrated in a sales brochure entitled Chill Jets.RTM. by TEC Systems.
FIG. 3 shows a section view of a prior art system representative of a forced contact system mounted on a first chill roller 115 of a printing press chill unit 114. In FIG. 3, a high pressure, air jet 321 is discharged from a nozzle 323, against the surface of web 110 and toward first chill roller 115. Web 110 is thereby forced into contact with the surface of chill roller 115. As a result, web "lift off" is reduced which squeezes the contaminated air from between the surface of web 110 and chill roller 115, thereby preventing condensate streaking.
Although such prior art forced contact systems, which discharge an air flow against the upper web surface (i.e., the surface which does not make contact with the chill roller) to force contact between the lower surface and the chill roll, are adequate in certain cases, the operation cost of such systems is high since such a system requires high pressure air for operation. In addition, the present inventors have determined that such forced contact systems are inadequate in applications using heavy, high quality webs, or in applications involving dense or thick ink coverage. The inventors believe that such inadequacy is probably due to the fact that forced contact systems are not effective in controlling web instability (induced by web flutter and web weave). The forced contact approach is therefore unable to keep the web uniformly in contact with the chill roller so that the contaminated air is allowed to come in contact with the chill roller and condense upon the chill roller. The limited effectiveness of such a forced contact approach is particularly apparent in high quality printing jobs where heavier webs are generally used.
Such forced contact systems may also contribute to environmental contamination by dispersing the contaminated air in an associated pressroom environment.
Thus, a non-invasive, low operating cost system is needed to reduce the formation of chill roll condensate in order to facilitate production of high quality printed images without sacrificing press speed or increasing web tension, and without exacerbating contamination of the pressroom environment.